


Crossing Borders

by Kaiosea



Category: World Trigger, ダイヤのA | Daiya no A | Ace of Diamond
Genre: Action, Crossover, Crossover Pairings, Crossovers & Fandom Fusions, Flirting, M/M
Language: English
Status: Completed
Published: 2015-12-15
Updated: 2015-12-15
Packaged: 2018-05-06 21:11:33
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,500
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/5430908
Author URL: https://archiveofourown.org/users/Kaiosea/pseuds/Kaiosea
Summary: <blockquote class="userstuff">
              <p>Tachikawa tries to protect an innocent civilian; the civilian, hardly innocent, does not want to be protected. (Jin’s Side Effect told him so, but Tachikawa wasn’t listening.)</p>
            </blockquote>





	Crossing Borders

**Author's Note:**

  * For [augusta_brie](https://archiveofourown.org/users/augusta_brie/gifts).



> For augusta_brie, this is a slightly late treat. I often think about the similarities between Daiya no Ace and World Trigger, which happen to be my two favorite ongoing animes, so I was excited to try my hand at writing something for you based on your letter. I hope you can find something you like in this highly unlikely crossover!
> 
> Thank you so much to [ouroboros](http://archiveofourown.org/users/ouroboros) for beta-ing, even though they're familiar with only one of the canons. Any errors regarding the two canons are my own.
> 
> For any readers familiar with only one of the canons, I hope that this might still make some sense out of context, but I do recommend both of these anime/mangas highly!

  


_Neighbor Emergence Area  
No Admittance_

 

Mid-afternoon on Saturday, Tachikawa gets called in on a potential alert. He wades through the weekend crowds until he reaches the edge of the boundary, aware of the civilian stares at his back as he crosses over. Though his Trigger is off, he knows how recognizable he is.

At a first glance there’s nothing to be seen or heard in the Forbidden Zone, and Tachikawa sighs, preparing to explore the area thoroughly. Likely a false alarm, since there’s one of them every few months. He dislikes these kinds of solitary patrols. Here, the silence rings in his ears; he’s not the type to cozy up too much to it, isn’t comfortable with it, doesn’t revel in it like Jin, and the difference between the confined areas and the bustle of the occupied, liveable areas of the city couldn’t be more stark. Even though one is inside the other, the existence of these enforced boundaries means that humans are still imprisoned here on Earth. So much as he’d prefer the growl in the distance to materialize into something real, an outlet for his frustration, he’s convinced he’s only imagining it.

Tachikawa rounds a corner, taking account of the row of abandoned houses, which is normal, and further down the street, a civilian walking with hands in his pockets, which isn’t.

He looks around Tachikawa’s age, probably late teens or early twenties, and he’s wearing a light T-shirt and a beanie. Glasses.

“Hey,” Tachikawa says, and when the guy fails to respond, he calls out to him more firmly.

The civilian swivels his head to look Tachikawa right in the eyes and then has the gall to swivel back, continuing to walk like he’s not even there.

“Oi,” says Tachikawa, a mild flare of annoyance heating his neck. “I’m Tachikawa of—”

At least the guy stops walking this time. “I know,” he says, pointedly not offering his own name. He pulls his hands out of the pockets of his sweatpants. “You’re from Border.”

“And you’re not allowed here,” Tachikawa says coolly.

“It was suggested I take a new way back my dorm today, so…” He blinks at Tachikawa nonchalantly, face giving away nothing.

Tachikawa is unfortunately accustomed to dealing with the annoying type of person who doesn’t like giving easy answers, so he can tell he won’t get anything out of this one. His best bet will be to escort him out of the area personally, and he heaves a grumpy sigh before jogging over to the civilian, who breaks his poker face, sporting a widening grin.

“What are you so happy about?” Tachikawa says, motioning for the guy to keep walking as he keeps pace. “Look sharp, I’m going to escort you back.” 

The civilian’s smile travels up to his eyes, squinting behind his glasses. “I dunno, really. What do you say we race?” 

“If you lead the way,” Tachikawa says immediately. He’s not in bad shape, and he has no intention of letting him out of his sight, so anything that gets them over the boundary and out of the Forbidden Zone faster is a win-win scenario.

The civilian nods and kicks up his heels, and they’re both off at a mid-fast pace. Swinging his arms in an easy rhythm, Tachikawa allows his grin to turn feral at the edges. No Neighbors in sight, racing a disagreeable gentleman with attractive eyes? He’ll take that over routine patrolling any day. If there’s a rumbling noise echoing from a few blocks away, it’s too quiet to be a significant cause for alarm.

Except it’s not really a race. Having someone this close in proximity radiates tension, but despite the feeling of something illicit in the air, they maintain a non-strenuous pace, taking a light jog at most. Tachikawa can’t stop himself from noticing the guy next to him: the easy bend of his knees, the flexion of his thighs. He’s breathing easy, like he’s comfortable with the laboured rhythm of running.

“You an athlete?” He gruffs out, and wants to bite his tongue for being so easily distracted. Silence will never be his strong suit.

The guy flashes him a grin, one that almost seems familiar. “Not a baseball fan?”

“I am.” Tachikawa returns the grin despite himself. “But we’ve been busy.” It’s been a while since he had the spare time to leaf through the usual magazines, though now that it’s been mentioned, he does look more familiar.

Pink forms at the middle of the guy’s mouth, and Tachikawa realizes he’s chewing gum while running. Not just chewing it, blowing a bubble: it pops loudly. “I bet. You know Jin, right?”

Tachikawa almost stops in his tracks. “Yeah.”

“Cool, I know him too…”

Tachikawa waits for more, but the guy just snaps his gum. What an unfortunate civilian, to be acquainted with someone like Jin. Or maybe Jin’s the unlucky one. Tachikawa looks up and grimaces at the clouds.

“I’m Miyuki,” the civilian says abruptly.

The name sounds familiar too, but Tachikawa can’t quite remember which team he belongs to, much less his position.

A less imaginary growl echoes from a distance, and they’re at a crossroads. Miyuki stops running, looks towards the sound, and flinches; Tachikawa doesn’t.

“Get out of here,” Tachikawa says, placing a hand at Miyuki’s shoulder, finding the muscle taut and tense. He attempts to show him to a pathway leading in the opposite direction to the noise, but Miyuki doesn’t budge.

“... supposed to keep going back to the dorm,” Miyuki mutters to himself.

“What?”

Miyuki straightens up. “I’m going this way.” Ignoring Tachikawa’s instructions, he turns towards the direction of the growl, instead of away.

“Hey, stop that!” Tachikawa runs up alongside him again. “You’re not a Border agent. I can take care of whatever this is, you get to safety.”

“How heroic. But all right, you can walk me to my dorm if you insist.”

They begin to jog in silence again, Tachikawa shoving down his instincts to search out and kill the probable Neighbor while he escorts this truculent civilian out of the Forbidden Zone. It’s not until they reach Miyuki’s school that the trouble materializes at last.

Blindsiding them, a Bamster charges through the gate into the campus before they make it all the way inside.

Instantly, Tachikawa switches his Trigger on. 

As his Trion body settles in, he curses, knowing that he’s barely a second too late to pull off a clean, safe kill. As he springs forward he sees a full crowd of students cowering back from the Neighbor, many carrying bats and gloves, and behind them an entire set of dormitory buildings lined up like weaponless soldiers. If the Bamster explodes here, even as it’s vanquished, it could cause dangerous damage to the buildings and the people inside.

This is civilian territory, and Tachikawa prefers to fight in a deserted town.

Meanwhile, Miyuki must have been holding almost everything back on their jog, because his flat sprint takes him inside school territory in a few seconds, keeping up with Tachikawa.

 _Get out of the way._ Tachikawa doesn’t have time to wonder where Miyuki will go. He needs to find somewhere without students, without inhabited buildings. It won’t be possible to draw the Bamster back to the Forbidden Zone, as they’ve come too far, and the Bamster is fully enticed by the prospect of a large group of people and the Trion they carry. Noticing Miyuki marshalling the students off to one side, Tachikawa tries to draw the Bamster’s attention in the opposite direction, using himself as a lure with some quick, flashy acrobatics.

Miyuki must be some kind of leader at the school, because the students begin to leave the area quickly. There’s a guy who looks like Miwa near the front of the crowd, lanky with black bangs over his face, and Miyuki’s mouth takes on a frenzied motion as he yells something at him, pointing with one finger. 

That’s when Tachikawa notices the empty stadium in the distance. Setting Miyuki’s face to a less prominent section of his mind, he begins to lead the Bamster in that direction, running in a zigzag pattern, the Bamster’s weight making the ground tremble beneath his feet. This, at least, is familiar. Every time the Bamster powers up, drawing energy into its mouth, Tachikawa takes a boost into the air so that its blast avoids the nearby buildings. At the increased capacity his Trion body functions, he only needs to draw and dodge three attacks before they’ve reached the diamond.

Once he’s inside the stadium, it’s an easy task to dispose of the Bamster. He feints to one side, only to ricochet back for a clean kill, the motion from planning to end taking less than ten seconds. He steps back, watching the armored giant leave a smoking hole in the ground. Piece of cake. Too bad about the damage to the outfield.

Second glances are for other people, but Tachikawa can’t help himself. He thinks about Miyuki, thinking he catches the sight of brown hair entering the stadium, and his grip on his Trigger slackens—only for a second, and if the warning had come on-time, or if he’d not relinquished his hold, the outcome of the day might have been different.  


**Warning: Gate activity detected.**

  
But combat is decided partially by luck, and there are idiosyncrasies of life that no one can predict. The warning is late, and Tachikawa will find out later that back at HQ, the system temporarily failed to register new Gate openings outside the Forbidden Zone.

So for now, he finds himself blasted from behind, dropping his blades along with his guard. It’s a rookie move that he’ll curse himself over later, unable to come up with a sufficient excuse. The Bamster’s leg has a lucky kick, and with such a loose grip, Tachikawa reacts too late, lunging after his swords only to see them fly to the other side of the diamond.

One second—five, ten seconds. That’s how long it will take him to reach his blades, by his own practiced mental calculations. No one ever said his mind wasn’t just as sharp as Jin’s, and Tachikawa doesn’t have the help of that annoying Side Effect or a Black Trigger, either. But then there’s the concurrent matter of the other two—no, three seconds. That’s how long until the new Bamster will be able to attack him again. And he needs the attack to focus on him, because he refuses to let it gallivant off to terrorize the campus.

He springs into action with single-minded focus, taking to the air before he’s finished the calculations. With his expertise and experience, he’s never truly unarmed. He lands near the top of the outfield fence, rattling the entire structure with a metallic clang. His fingers stick through the chain links, and he clings to the fence. Once the Bamster notices where he’s gone, Tachikawa’s already climbing. Its first attack misses him, though it does blow a giant hole in the fence. If he can make it 180 degrees around to the other side, he’ll procure his blades without difficulty.

Miscalculation.

Tachikawa hears its attack building too quickly, the high-pitched whine making his pupils blow wide in frantic anticipation. He climbs the cage, not wanting to redirect the monster’s attention to the nearby buildings with the civilians hidden inside, but in the split second he has to climb he knows he won’t evade the monster’s range, knows he’s about to get hit and bail out—

It’s been too long since he’s thought of himself as fragile, and he’s not about to start now.

_Crack!_

Something spins into the monster, splitting its armor like a stroke of lightning. — _The work of a Trigger user?_ But he doesn’t feel an agent nearby. That kind of power, he’d have sensed if it were in the area. Blood rushes into Tachikawa’s head and he flips off the back with relief, oxygen returning to his brain as he catches the rogue projectile on its ricochet off the monster’s armor, wondering what exactly is the object that attacked the Bamster for him. He spares a glance to the weight in his hand.

A baseball spins on the tips of his fingers.

Tachikawa reassesses his surroundings in a blink, noticing the presence of other people on the field. One is Miyuki. The other, he doesn’t know.

Miyuki’s voice is hollering something, and with the wind whipping his ears, it takes Tachikawa until his feet hit the ground to understand what he’d said:

“Straight over the middle!” In his hurry to retrieve his blades, Tachikawa has to blink before realizing the command isn’t meant for him. 

He barely catches the motion at the corner of his eye—but then again, his eye for this sport has always been good, and it’s not without admiration that he instantly catalogues the motions of the dark-haired boy on the mound, the one who resembled Miwa earlier. Not-Miwa’s shoulder twists back behind his head, and he lunges forward in a liquid motion, his arm with impeccable follow-through. From there it’s a quick wink over to Miyuki, clad in batting helmet and gloves, fire in his eyes as he waits for the ball, studying it like a pro. He swings like a whirlwind, connecting with another loud crack.

The ball flies forward to hit the Bamster in its eye, and the machine rears up with a screeching mechanical roar. That’s Tachikawa’s signal to go. His blades are in his hands. He exhales all of his breath, takes off at a full sprint, and jumps, landing on the monster’s head.

Twin swords swipe the life source inside its mouth, and Trion begins to leak from its body, signalling the beginning of its death.

“What the hell,” Tachikawa says. Once he’s on fair ground again, he raises his voice to ring over the Bamster’s groan and yells, for amusement’s sake, to the pitcher: “Play ball!”

Swiveling his head, the pitcher nods to him once, then turns back to Miyuki and winds up, the Bamster’s whine still dying down. The ball shoots through the air like a bullet once again, zooming straight into the strike zone, where its batter waits with the coiled strength of an animal. 

One foot out of the box, Miyuki swings.

The fence rings from the impact of the Bamster’s monstrous body as it crashes, the ball perfectly meeting its target. It vibrates Tachikawa’s toes through the ground, and he takes the air like he owns it, jacket swooping out behind him as he flies through the air to plant the final blow. His blades slice the monster’s weak spot like it’s nothing, and he turns his back on it and jumps off its back, not looking over his shoulder but feeling the blowback of the ensuing explosion.

“Home-run,” Not-Miwa says placidly, in the dusty silence after the explosion has finished. 

If the outfield hadn’t been damaged beyond repair before, it certainly is now.

Miyuki doubles over and Tachikawa experiences a horrified moment of panic, catching himself leaping forward, making it past the mound before realizing that Miyuki’s actually _laughing._

“One second,” Miyuki says to Tachikawa, wiping under his eyes. He pulls off his helmet. There’s sweat trickling down his neck, his hair’s mussed up, and Tachikawa can smell him now—spent as if he’d finished a marathon, or even a marathon round of something else (Tachikawa isn’t going to think about it). Tachikawa watches his thighs tremble as he stumbles over to the mound in a weak imitation of his earlier jogging gait.

Miyuki claps Not-Miwa on the back and says a few words, too quietly for Tachikawa to catch. The pitcher puffs out his chest, and Miyuki hits him on the back again, making him let out his breath before he picks up his fallen glove and exits with a light jog up the hill. Not-Miwa runs back to the dorms without introducing himself.

Seems to be a pattern with the students from this school.

After powering his Trigger off, Tachikawa brushes dust off his sleeves. “What took you so long?”

“I had to warm him up,” Miyuki says. “Wouldn’t want him to get injured.”

“You knew you had that kind of time?”

Miyuki shrugs. “What can I say, I’ve always been a little perceptive.”

Something pings in his memory. Tachikawa thinks about it for a second, but no longer. “You did a good job. If your Trion levels are high enough, I think you could be recruited.”

Miyuki laughs. “My levels are low as hell, but I’d rather play baseball anyways.”

Tachikawa tilts his head. “Fair enough.” Deciding to press his luck, he says, “I have to go soon, but if there’s other trouble, and the scanners don’t pick it up, I’ll give you my number.”

“How kind of you,” Miyuki says, snickering. He doesn’t have his phone on him, but says he’ll remember Tachikawa’s number. Tachikawa is impressed when Miyuki recites it back to him after hearing it once.

Standing this close to him, Tachikawa is pondering how they must be almost exactly the same height when Miyuki takes a step closer, challenging him with his stare.

“Well?” Miyuki says. “I thought you Border guys were used to making the first move.”

Without a second thought, Tachikawa leans forward, mouth close enough to kiss him—but instead, he just stays in that position. To his credit, Miyuki doesn’t break eye contact.

They’re just there, breathing on each other in the middle of destroyed grass and sand and a little crushed metal fencing.

He swipes his mouth along Miyuki’s top lip, slightly worrying it between the sharp edge of his teeth, but he still doesn’t go in for a full kiss. His palm molds over the nape of Miyuki’s neck, and Tachikawa tilts his head back to see Miyuki’s jaw loll open, seemingly pliant, though his eyes might say otherwise. His tongue looks rough and pink inside his mouth.

No gum.

Before they get any further, Tachikawa’s alert beeps in his pocket. Tilting his head forward again, he freezes.

“Shit,” he says. He speaks right against Miyuki’s lips. “I have to report back.”

“So soon?” Miyuki says, narrowing his eyes. Tachikawa has no doubt the sultry look is intentional.

Outside of work, Tachikawa has no problem leaving things unfinished. It just means there’s empty space to come back for more. He takes his hand away from Miyuki’s neck. It’s probably for the best.

They take an awkward step back from each other, and Tachikawa notices Miyuki’s quickened breathing with pleasure.

“Hold on,” Tachikawa says. Sense returning to him, he realizes that he doesn’t recognize the field they’re standing on, struck by a sudden doubt. “What team do you play for?”

Miyuki’s nose wrinkles, like he’s suppressing a smile, but he doesn’t withhold information this time. “Seidou High. Catcher.”

Ah, high school. That’s explains it. Tachikawa remembers the correct, old magazine now—he hadn’t been thinking non-professional. “You know where you're headed in a few months?”

Miyuki shrugs and says, “Nah. We can’t all predict the future.”

 _In fact, only one person can,_ Tachikawa thinks. He supposes Miyuki’s talent will likely carry him far. “Your mouth’s going to get you in trouble someday,” he says. Nothing like a friendly warning as a parting message.

Miyuki smiles and lifts his eyebrows, looking more unreasonably familiar by the second. “Yeah, I hope so.”  
  
  
  
  
—  
  
At HQ, Tachikawa catches sight of Jin at one of the common area couches and saunters over.

“Back so soon?” Jin says.

“I already clocked the whole thing with the big guys,” Tachikawa yawns. “Where were you?”

“Other side of town, handling a disturbance.”

“Ah, of course you were.”

“How was your date?” Jin says. “That is, if you feel like telling me some things I already know.”

Tachikawa says, “Don’t tell me about your Side Effect—”

“Why, you’re not calling me a pervert, are you? I’d never use my powers for spying.”

Tachikawa blinks and folds his arms. “You and your setups,” he says in a mild voice.

Jin shrugs and takes his phone out of his pocket, setting it on the table. “You picked a bad time to talk.”

Tachikawa knows better than to say something idiotic like, “But you’re not busy right now.”

Jin’s phone rings, of course. Without looking at it, he says, “It’s my cousin. Gotta take this.”

“You have a cousin?”

Jin frowns. He taps his phone and turns his back, pressing his free hand to his ear right in time to block out the noisy conversation of some passerby. “Didn’t I tell you you’d thank me later?” He says impatiently, and it’s like he’s speaking to both Tachikawa and the person on the other line.

Spreading his arms over the top of the couch, Tachikawa chuckles to himself. He closes his eyes. Laying his head back, he thinks about his hand at the nape of Miyuki’s neck, and the empty space of the future.

**Author's Note:**

> Thank you for reading this seriously wacky crossover, if you made it this far! If you have any thoughts on Daiya, World Trigger, I'm more than interested in hearing it. I love these canons so much :D


End file.
